This invention relates to fuel injectors for diesel engines and more particularly to a fuel injector having a metering and an injection mode of operation and an intensifier piston which defines an upper chamber, a middle chamber and a metering chambering wherein the motion of the piston cooperates to selectively maintain a vacuum or low pressure level within the middle chamber.
Diesel fuel injection systems often require that the fuel to be injected into the respective cylinders of the diesel engine be at pressures that may be in excess of 15,000 psi. Fuel systems which pressurize the fuel carrying components, upstream of the injector, at high pressure levels exhibit fuel leakage and engine horsepower drain. It is known that by utilizing injectors having an intensifier piston permits operation at lower upstream pressures. As an example, if it has been determined that the injection pressure of the fuel injector must be 18,000 psi, an intensifier piston having an intensification ratio of 3:1 would reduce the pump requirements from 18,000 to 6,000 psi. One such fuel system incorporating an intensifying piston is disclosed by Walter et al in the commonly assigned patent application U.S. Ser. No. 217,297, filed Dec. 17, 1980 now U.S. Pat. No. 4,426,977.
Broadly speaking, an intensifier piston includes a large face area upper body portion or member connected to a smaller face area lower body portion member. The intensifier piston is reciprocally situated, in form fitting contact, within a stepped bore of the injector housing. The geometry of the intensifier piston, as well as its reciprocating motion within the injector housing, define an upper chamber, proximate to and above the upper body portion. The upper chamber is adapted to receive pressurized fuel which is transmitted to a first pressure receiving surface of the upper member. The piston and housing also form a middle chamber that is situated between the upper portion and the large diameter portion of the stepped bore. A metering chamber is situated below a second pressure receiving surface on the lower body portion of the intensifier piston. During the operation of this type of injector, pressurized fuel is transmitted to the upper receiving surface of the intensifier piston, because of the relationship of the areas of the first and second pressure receiving surfaces, the intensifier piston inherently performs a pressure amplification function. The theoretical value of the intensification ratio is determined by the ratio of the areas of the first and second pressure receiving surfaces. As will be discussed below, the effective intensification ratio of these injectors is often less than the theoretical ratio.
Walter et al in U.S. Ser. No. 217,297, further incorporates a laminar flow restrictor situated in a piston. The purpose of this restrictor is to restrict the fluid flow from a downstream accumulator back into the upper chamber. The restrictor also permits the piston to move upward as the pressure above the piston is reduced due to the operation of a cooperating fuel pump.
It has been found that the injector of Walter et al and similar injectors display an injection pressure which is less than expected. The reduction of the injection pressure arises because of inefficiencies in its operation due to the additional work necessary to compress fuel within the middle chamber and also because of the additional work lost by pumping fluid through long lines between the injector and the fuel tank. This work generates unwanted heat within the fuel injector and drains needed horsepower from the engine.
These deficiencies are solved by the present invention by trapping a small amount of fuel under the primary or upper body member or portion of the intensifier piston so that as the intensifier piston rises during its metering mode of operation, the fluid pressure within the metering chamber is dropped to its vapor pressure. Consequently, during a subsequent injection mode of operation, there is no fliud to be pumped out of this chamber or to be compressed and therefore, no unnecessary work is expended. The invention further utilizes a check valve which prevents unwanted fluid from entering the middle chamber. The check valve further serves to restrict the flow of fuel from an accumulator into the upper chamber thereby creating a pressure differential across so that the metering mode of operation can start promptly.
More specifically, the fuel injector comprises a housing having a first port that is adapted to receive pressurized fuel from a first pressure source and a second port which is similarly adapted to be connected to a second source of pressurized fuel which is generally maintained at a pressure level less than the pressure level of the first source. The housing further includes a stepped bore having a larger first bore that is linked to a narrower second bore. The housing additionally includes a first dump port situated on the first bore and a second dump port that is situated on the second bore. An intensifier piston that is responsive to the pressure differential thereacross is provided for reciprocatively moving within the stepped bore and for selectively uncovering the first and the second port. The intensifier piston and the stepped bore cooperate to provide a plurality of variable volume chambers such as an upper or primary chamber, a middle or an inner chamber and a lower or metering chamber. The injector further includes a nozzle which extends from the housing and is operatively connected in fluid communication with the lower or metering chamber for injecting a predetermined quantity of fuel therefrom in correspondence with a motion of the intensifier piston. The fuel injector further includes a first fuel passage which connects the second port with the lower chamber having positioned therein a first check valve to inhibit the flow of fuel from the lower chamber through to the second port. The injector additionally contains a second fuel passage which connects the middle chamber to the second dump port and a third fuel passage having lodged therein a second check valve for inhibiting the flow between the second port and the first port. In addition, the fuel injector housing further includes a fourth fuel passage for connecting the first port to the middle chamber.
It is an object of the present invention to improve the efficiency of operation of diesel fuel injectors having intensifier pistons. It is a further object of the present invention to provide a fuel injector having a middle chamber that is selectively maintained at a low or vacuum pressure. It is a further object of the present invention to provide a fuel injector of simple construction and one that does not require sophisticated manufacturing operations.
It is a further object of the present invention to provide a diesel fuel injector having an effective intensification ratio which approximates the theoretical intensification ratio. A further object of the present invention is to provide a fuel injector having an injection and metering mode of operation which is characterized by a rapid transition between the modes of operation.
A feature of the present invention is the creation of a vacuum pressure within the inner or middle chamber. This vacuum pressure is not formed until the metering mode of operation has begun. More specifically, a small amount of fuel is trapped within the middle chamber during the injection mode of operation. During the metering mode of operation, that is, when the piston is caused to rise, the fluid pressure within this chamber is dropped to its vapor pressure. Consequently, during subsequent injection modes of operation, that is, when the intensifier piston is moving downward, no work can be expended in compressing fluid or in pumping fluid out of the middle chamber and therefore, no unwanted pressure forces are developed which tend to reduce the intensification ratio.
An advantage of the present invention is that it may be packaged and adapted to all sizes of diesel engines with virtually no engine modification. By increasing the efficiency of operation of the injector, the size and the flow capacity of cooperating distributor pumps, which supply fuel to each of the injectors, is correspondingly reduced.
According to the specific embodiment illustrated in the drawings of this application and discussed in detail below, the present invention comprises :